To keep the skin moisturized and soft, water in the horny layer is known to be important. Maintenance of availability of such water is considered to rely upon water-soluble components contained in the horny layer, namely, free amino acids, organic acids, urea or inorganic ions.
With the foregoing in view, it has been strived to lessen or prevent skin roughness by adding such substances either singly or in combination in medicinal external skin care preparations or cosmetics.
Besides the substances mentioned above, numerous humectants having high affinity with water have been developed and are used for lessening skin roughness.
However, when such humectants are applied to the skin, they remain on the skin horny layer and act to supply water to keratin. Moreover, their effects are temporary. They cannot therefore be considered to fundamentally improve the water-retaining ability of the horny layer and to basically prevent or cure skin roughness.
As an external skin care preparation effective in basically improving the water-retaining ability of the horny layer, the present assignee already proposed an external skin care preparation comprising an amide derivative represented by the following formula (5) (Japanese Patent Laid-Open No. 228048/1987, now Japanese Patent Publication No. 42934/1989): ##STR2## wherein R.sup.1b represents a linear or branched, saturated or unsaturated hydrocarbon group having 10 to 26 carbon atoms and R.sup.2b represents a linear or branched, saturated or unsaturated hydrocarbon group having 9 to 25 carbon atoms.
Further, the present assignee also proposed external skin care preparations comprising amide derivatives having similar effects inter alia in Japanese Patent Laid-Open Nos. 216812/1988, 218609/1988, 222107/1988, 227513/1988, 29347/1989 and 31752/1989.
Moreover, the present assignee found that cosmetic hair care formulations containing amide derivatives represented by the below-described formula (6) can penetrate into the hair to effectively protect the same, can prevent chlorosis of the skull and can improve the touch-feeling of the hair, and also proposed them in Japanese Patent Laid-Open No. 9913/1989. ##STR3## wherein R.sup.1b represents a linear or branched, saturated or unsaturated hydrocarbon group having 10 to 26 carbon atoms, R.sup.2b represents a linear or branched, saturated or unsaturated hydrocarbon group having 9 to 25 carbon atoms, and X represents (--CH.sub.2 )--.sub.n in which n stands for an integer of 2 to 6.
Although these amide derivatives bring about the above-described excellent advantageous effects, they have properties such as a high melting point, high crystallinity and low solubility in bases so that when added to external skin care preparations and cosmetic hair care formulations, they still involve unsolved problems in compatibility, interaction and the like.